Immunoassays such as radioimmunoassays (RIA) and enzyme immunoassays (EIA) are useful methods for determining the presence, identity, and amount of one or more analytes of interest in a sample. Many immunoassays immobilize an antibody specific for an analyte of interest on a solid phase, e.g., a microplate or bead; binding between the bound antibody and analyte present in a sample is detected, such as through the use of a sandwich assay. Other immunoassays immobilize the analyte; these immunoassays can be referred to as solid phase antigen or solid phase analyte immunoassays. In solid phase analyte immunoassays, the solid phase analyte competes with analyte present in a sample for binding to an antibody specific for the analyte. Typically in such solid phase analyte assays, the antibody is detectable in some manner, e.g., it is labeled, such as radioactively, fluorescently, luminescently, or enzymatically (e.g., an enzymatic reaction occurs in the presence of an appropriate substrate, resulting in a color change) labeled, or its presence is detected via a secondary antibody that itself is labeled.
In order to determine if a particular sample contained more than one analyte of interest, previous methods typically employed separate solid phase components (e.g., separate microplates or sets of microbeads), with each solid phase component containing a bound antibody specific for one particular analyte, or with each solid phase component containing a single type of bound analyte. Such a need for separate solid phase components for each analyte renders multi-analyte assays expensive, technically complex, and time-consuming. There exists a need for an efficient and relatively inexpensive analyte detection method that can rapidly determine the presence and/or amount of one or more analytes, including analytes such as drugs of abuse, in a sample.